Hydraulically operable extendable boom

ABSTRACT

An extendable boom formed of three telescopically received boom sections with a pair of hydraulic piston and cylinder assemblies in side-by-side relationship on the boom interior with one cylinder being axially movable with respect to the other form retracted to extended position for extending the boom and with the piston of one cylinder being connected to the inner end of the inner boom section and the cylinder of the same assembly being connected to the inner end of the middle boom section with the piston of the second hydraulic assembly being connected to the middle section and with the cylinder of the second assembly being connected to the outer boom section.

United States Patent Edwin E. Milner, Jr.

Williamsburg;

William R. Pierce, Richmond, both 01' Va. 35,343

May 7, 1970 Oct. 5, 1971 Baker Equipment Engineering Company Richmond, Va.

inventors Appl. No. Filed Patented Assignee l-IYDRAULICALLY OPERABLE EXTENDABLE Primary Examiner-Edgar W. Geoghegan AttorneyMason, Fenwick & Lawrence ABSTRACT: An extendable boom formed of three telescopically received boom sections with a pair of hydraulic piston and cylinder assemblies in side-by-side relationship on the boom interior with one cylinder being axially movable with respect to the other form retracted to extended position for extending the boom and with the piston 01 one cylinder being connected to the inner end of the inner boom section and the cylinder of the same assembly being connected to the inner end of the middle boom section with the piston of the second hydraulic assembly being connected to the middle section and with the cylinder of the second assembly being connected to the outer boom section.

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ATTORNEYS PATENIED 0m 5 1971 SHEET 6 [IF 6 i [ii HYDRAULICALLY OPERABLE EXTENDABLE BOOM This invention is in the field of telescopic boom constructions and is specifically directed to the field of hydraulically actuated booms formed of telescopically oriented plural boom sections.

A great deal of effort has been made by those skilled in the art in recent years to provide advances in the art of telescopically extendable boom constructions. Efforts along these lines have resulted in a wide variety of boom constructions such as cable actuated boom extension drives, internal and external hydraulic system extension drives and hybrid combinations of both cable and hydraulic systems. Unfortunately, the prior systems have suffered from a number of disadvantages such as high cost in initial construction and maintenance due to the complexity of many of the devices. Moreover, the complex nature of many of the prior known devices have required extensive skill on the part of the operator and has provided an inherent source of maintenance problems. It is desirable that telescopically operable boom constructions be economical to construct and maintain and also be easy to use and operate. Simplicity and reliability of construction are of prime desirability. Such boom constructions should be easily extended and retracted with a minimum of time and effort and should be as friction free as possible. The subject invention provides a remarkably reliable and easy to use boom construction which occupies a minimum of space while retaining a maximum of lift capability. Moreover, the entire hydraulic system for the boom is located on the interior of the boom in a space-saving manner.

The great advantages of the subject invention over the prior art devices will be more fully appreciated upon achieving a full understanding of the preferred embodiment of the invention which is illustrated in the drawings and in which:

FIG. 1 is an elevational view of the preferred embodiment of this invention illustrating different boom positions;

FIG. 2 is a bisecting sectional view of the boom construction of the preferred embodiment;

FIG. 3 is a sectional view taken along lines 3-3 of FIG. 2;

FIG. 4 if a sectional view similar to FIG. 2 but illustrating the boom in an extended condition;

FIG. 5 is a sectional view taken along lines 55 of FIG. 2;

FIG. 6 is a sectional view taken along lines 66 of FIG. 2;

FIG. 7 is a bisecting sectional view of one of the control cylinders employed in the preferred embodiment;

FIG. 8 is a sectional view taken along lines 8-8 of FIG. 2;

FIG. 9 is a hydraulic schematic of a first control circuit for controlling the preferred embodiment; and

FIG. 10 is a hydraulic schematic of a second control system.

DESCRIPTION OF THE PREFERRED EMBODIMENT The preferred embodiment of this invention comprises an extendable boom which is mounted for pivotabl movement in a vertical plane about a pivot axis 21 of a rotary turret 23 which is normally supported on a truck or other vehicle 22 of conventional design.

Boom 20 is formed of an inner section 24 which is directly pivoted to the pivot axis 21 at its lower end, a middle section 26 telescopically received in the inner section 24 and an outer section 28 which is telescopically received within the middle section 26. The terms "inner and outer" are used with respect to the positions of the sections and other structural elements with respect to the turret when the boom is extended; for example, the inner" section 24 is more closely adjacent the turret than is "outer section 28.

It should be noted that all of the boom sections are of rectangular cross-sectional configuration and middle section 26 and outer section 28 are reciprocable with respect to each other and with respect to the inner section 24.

Turning now to FIG. 8, It should be noted that inner steel section 24 is formed of a top panel 32, a bottom panel 34 and side panels 36 and 38. Similarly, the middle section 26 is formed of steel and includes top and bottom panels 40 and 42 respectively and side panels 44 and 46. In like manner, the

outer section 28, which is made of fiber glass, comprises top and bottom panels 48 and 50 respectively and side panels 52 and 54.

A modified cylindrical Teflon bearing 56 in the shape of a cylinder that has been out along a plane perpendicular to its axis (FIG. 2) is mounted on the inner end of the top panel 40 of middle section 26 for engagement of its planar portion with the bottom surface of top panel 32 of the inner section 24 and second cylindrical Teflon bearing 57 is mounted in panel 40 for engagement with the upper side of upper panel 48 of the outer section 28. In like manner, bottom panel 50 of outer section 28 is engaged by a cylindrical Teflon bearing 59 mounted in bottom panel 42 of the middle boom section 26. Addi tionally, a top cylindrical Teflon bearing 60 is mounted on the outer end of top panel 32 of the inner section 24 and engages the top panel 40 of the middle section 26. Similarly, a cylindrical bottom Teflon bearing 62 is mounted in the bottom panel 34 of the inner section 24 for engagement with the bottom panel 42 of middle section 26 and a cylindrical Teflon bearing 58 is mounted on panel 42 for engagement with panel 34. Consequently, middle section 26 is reciprocal within inner section 24 with a minimum of effort but is restrained from lateral movement.

The outer section 28 is mounted in a similar manner within the confines of the middle section 26 by virtue of upper and lower Teflon bearings 57 and 59 respectively mounted in panels 40 and 42 of the middle section and Teflon bearings 68 and 70 which are respectively mounted in the top panel 48 and bottom panel 50 of the inner end of the outer section. Therefore, it will be easily appreciated that the entire telescoping boom construction is movable from a retracted to an extended position with a minimum of effort and work being required for such movement by virtue of the low friction characteristics of the various Teflon bearings employed in the boom.

Reinforcing members 69 and 71 are respectively attached to the side panels 36 and 38 of inner boom 24 for providing required strength at the bottom of the boom. The boomsupporting pivot shaft 21 is supported in turret bracket plates 72 and 74 between which the inner end of the boom is received on the pivot shaft 21.

Additionally, a hose reel assembly 76 is mounted on the turret ans has two hydraulic hose elements 78 for selectively supplying pressurized hydraulic control fluid to a pilot valve for controlling one of two hydraulic cylinder means in the boom for extending and contracting the boom in a manner to be discussed hereinafter. It should be understood that hydraulic pressure from a conventional source is selectively supplied to one of hose elements 78 in a conventional manner through one of two inlet lines 80 from a conventional hydraulic pump and control valve assembly controlled by the operator.

A boom elevating and lowering cylinder assembly 82 is connected to a transverse shaft 84 extending across the base of the turret and has a piston rod 86 having its outer end connected by a pin 87 to a bracket 88 extending downwardly from the bottom panel 34 of the inner section 24. Actuation of cylinder 82 serves to pivot the entire boom 20 about the axis of shaft 21 in a vertical plane to various angles of inclination as shown in FIG. 1.

Extension of the telescoping sections of the boom is enabled through the provision of first and second hydraulic cylinder members 90 and mounted within the confines of the boom on the interior of the outer section 28 when it is retracted within middle section 26. The innermost end of cylinder 90 is connected to the middle section 26 by four pins 102 which extend through connector blocks 104 welded to the interior of the side panels 44 and 46 of the middle section 26 and blocks 104 welded to the cylinder. A slide bearing block 105 is fixed to the outer end of cylinder 90 and has an arcuate polished bearing surface riding on the outer surface of cylinder 100 which is also polished to provide a low friction contact between the bearing block and the cylinder. Additionally, it should be noted that the upper surface of block 105 is in sliding contact with the lower surface of the upper panel 48 of the outer boom 28. Piston rod 106 extending from cylinder 90 is connected on its end to inner boom 24 through a rod extension block 109 welded to a cylindrical sleeve 107 having a bore through which a pin 108 extends and which is connected to walls 36 and 38 on each end. All movement of cylinder 90 is consequently immediately conveyed to the middle section 26 so that cylinder 90 and middle section 26 move as a unitary unit.

Cylinder 100 is connected on its innermost end by blocks 1 l1 welded to the cylinder through which pins 1 extend and which also extend into blocks 112 connected to the inner end of the outer boom 28. A piston rod 114 extending from cylinder 100 is connected through a rod extension block 115 welded to a sleeve 117 to a pin 116 connected to the inner most end of the middle boom section 26. Moreover, the outer end of cylinder 100 is connected to a bearing block 119 which rests on the lower wall 50 of the outer boom 28. Consequently, actuation of cylinder 100 to extend the rod 114 with respect to cylinder 100 serves to move the outer boom 28 outwardly with respect to the middle boom 26 in an obvious manner.

Hydraulic power for actuating the cylinders 90 and 100 is provided through a pair of inlet lines 118 and 120 extending, in the control circuit embodiment of FIG. 9, from a conventional four-way three-position control valve (not shown) controlled by the operator. Line 118 is connected to the rod extension block 109 by a right angle connector block fitting 122 welded to the top surface of rod extension 109 as shown in FIG. 7. Communication between line 118 and an axial bore 124 in rod extension 109 is provided by a right-angle passageway 126 formed in connector block 122 and communicating with a passageway 128 extending outwardly from axial bore 124 in rod extension 109. It should be noted that the connection between line 118 and connector block 122 is made through a pilot check valve 130 from which an intermediate line 132 extends for connection to line 120.

Line 120 is connected to a lower right angle connector block fitting 134 which has a right angle passageway 136 communicating with a radial bore 138 extending from an axial bore 140 which extends the entire length of piston rod 106. An axially aligned steel tube 142 extends the entire length of axial bore 140 with the interior of tube 142 communicating with the axial bore 124 formed in the rod extension 109. Consequently, pressure line 118 communicates with the interior of the tube 142 and line 120 solely communicates with the portion of bore 140 external of tube 142.

A piston 144 is connected to rod 106 on the interior of cylinder 90 and tube 142 extends through piston 144 so that line 118 is in communication with the space on the side of piston 144 opposite the side to which rod 106 is connected. Similarly, a radial bore 146 extends from the axial bore 140 so that line 120 is in communication with the space to the left of piston 144 (the rod side) within cylinder 90. Consequently, hydraulic pressure introduced through line 118 will tend to move cylinder 90 outwardly with respect to red and piston assembly 106, 144 while pressure introduced through line 120 will tend to retract the cylinder 90 inwardly to the left as viewed in FIG. I.

The outer or head end of cylinder 90 is provided with a right-angle fitting 148 which has a passageway 149 communicating with that portion of the interior of cylinder 90 to the right of piston 144. A steel tube 160 is connected to the right angle outlet fitting 148 so that its interior passageway is in communication with passageway 149. Tube 160 extends along the bottom of cylinder 90 as viewed in FIG. 2 back to a position adjacent the innermost end of cylinder 100 at which point tube 160 is connected to a conventional check valve 162. The other side of check valve 162 is connected to a pilot actuated control valve 164 by a short tubular conduit 166. Similarly, a second right-angle outlet fitting 168 is connected to the inner end of cylinder 90 and has an interior passageway 170 communicating with the rod side of the interior of cylinder 90. A

tubular steel conduit 172 extends from fitting 168 to another inlet on the pilot control valve 164. The pilot control valve 164 has two outlet lines 174 and 176. Line 176 is connected through a pilot check valve 221 to a right-angle connector block fitting 222 which is analogous to fitting 122 and which has an interior passageway communicating with a radial passageway 224 extending outwardly from an axial bore in a rod extension 1 15 connected to piston rod 1 14 extending from cylinder 100. Line 174 is connected by line 223 to pilot check valve 221 and to a connector block fitting 234 which has a right-angle passageway communicating with a radial bore extending from an axial bore 240 which extends the entire length of rod 114. A tube 242 extends the length of rod 114 to communicate with a radial bore 224 in the same manner that tube 142 communicates with the bore 128 formed in the rod extension 109. The connections between tubes 174 and 176 at the fittings on cylinder 100 are analogous to connections to cylinder with steel tube 242 extending through the piston 206 of cylinder so that tube 176 is in direct communication with that portion of the cylinder on the right side of a piston 206 opposite rod 114. Similarly, line 174 is in direct communication with the portion of cylinder 100 to the left of the piston 206 by virtue of a radial bore 246 extending outwardly from bore 240 adjacent the piston 206 as shown schematically in FIG. 9. Both of the hose connections 78 from reel 76 are connected to the remote controlled valve 164 for controlling the valve.

It should be noted that remote controlled operator valve 164 is attached to the innermost end of the bottom plate 42 of the middle boom 26 and consequently moves with the boom as will be clear from inspection of FIG. 2. The cylinder 90 is connected to the pilot valve 164 by the steel lines 172 and and cylinder 90, the aforementioned lines and the middle boom 26 all move as a unitary unit in an obvious manner. Therefore, there is no need for flexible connections between cylinder 90 and the valve elements on boom 26. In fact, the only flexible connections required are those of hose members 118,120 and 78.

A drop-down pulley 250 is mounted in the end of the outer boom by means of a pivot frame comprising first and second spaced pivot plates 252 and 254 between which pulley 250 is mounted for rotation by means of an axle 256 extending between the plates. Pivot plates 252 and 254 are mounted for rotation about a main pivot shaft 258 which extends transversely perpendicular to the sidewalls 52 and 54 of the outer boom section 28 and is mounted in a box frame comprising steel sidewalls 260 and 262, a top wall 264 and a slotted bottom wall 266 is fixed in the outer end of the interior of the outer boom 28. An arcuate slot 270 is provided in plates 252 and 254 into which a retaining shaft 272 extends. It should be noted that the outer end of the middle boom section 26 extends beyond the outer end of the outer boom when all of the booms are in their retracted positions as shown in FIG. 3 and the outermost end of the bottom plate 50 of the outer boom is slotted to permit the pulley 250 and pivot plates 252, 254 to assume the position illustrated in FIG. 4. A cable from a conventional boom-tip winch 282 extends over pulley 250.

A cycle of operation of theextendable boom under the control of the hydraulic system illustrated in FIG. 9 will now be discussed. Cylinder 90 must be fully retracted in order for cylinder 100 to be operated with positive control. Therefore, it will be assumed that the boom is in its fully retracted position illustrated in FIG. 2 with cylinders 90 and 100 being fully retracted.

Pilot valve 164 is a conventional spring centered, closed center spool piston-type four-way three-position valve (Valve No. 1255 manufactured by Racine Hydraulics of Sarasota, Fla. has been found to be satisfactory for this valve). An internal spool is positioned in accordance with pilot pressure signals introduced into valve 164 by either of pilot pressure lines 78. The nature of the pilot pressure signals introduced via lines 78 is determined by the position of the piston 284 of a four-way three-position manually operable control valve 286 operated by the operator of the machine. Valve 286 is connected to a pressure source line 288 and an exhaust line 290. When control piston 284 is in the position shown in FIG. 9, both the inlet lines 288, the exhaust line 290 and pilot pressure lines 78 are all in communication via an open center passage 291 in the piston 284. Since the pressure in both pilot pressure lines 78 is equalized, spool centering springs (not shown) in valve 164 position the valve spool in valve 164 in an intermediate or blocking position in which lines 174 and 176 are blocked. Consequently, piston 206 and cylinder 100 will be held in fixed position with respect to each other. However, valve piston 284 can be moved downwardly from the FIG. 9 position so that the pressure line 288 communicates with the upper line 78 and the lower line 78 communicates with the exhaust line 290. Therefore, pressure from line 172 enters that portion of cylinder 100 to the left of piston 206 via passageways 240, 246 etc., and exhaust flows out through line 166 etc., from that portion of cylinder 100 to the right of piston 206. When control pressure is supplied to the upper line 78 so that the spool of valve 164 is in its lower position, only lower line 78 is connected to exhaust line 290; however, upward movement of valve piston 284 provides for a reverse connection with the pressure being connected to lower line 78 and the exhaust line 290 being connected to upper line 78.

When it is desired to extend the boom, power pressure line 120 is connected to a main source of pressure by the main pressure four-way three-position valve at the operators control point and line 118 is connected to exhaust. Valve 164 is actuated to connect line 174 with line 160 and to connect line 176 to line 172. Pressure is introduced to the left of piston 144 in cylinder 90 by way of line 120, the cylinder 90 remains retracted and pressure from the portion of cylinder 90 to the left of piston 144 flows through line 172 to directional control valve 164 and is directed to line 176 to flow outwardly through tube 242 through piston 206 to enter the cylinder 100 to the right of piston 206 and consequently initiate expansion of cylinder 100. Expansion of cylinder 100 is enabled by virtue of the fact that hydraulic fluid to the left of piston 206 flows inwardly through radial passageway 246 and bore 240 to be exhausted through line 174 which is in communication with exhaust line 166. Consequently, the exhausted fluid flows through line 160 into that portion of cylinder 90 to the right of piston 144 and is then exhausted through the steel tube 142 back through the line 118. However, when cylinder 100 reaches its maximum position of extension, or any desired intermediate position for that matter, valve 164 is moved to its neutral position with lines 174 and 176 then being blocked to maintain cylinder 100 and piston 206 in their then present position. Cylinder 90 can then be extended to any desired position by introducing pressure through line 118 into that portion of cylinder 90 to the right of piston 144. Outward movement of the cylinder 90 (to the right as viewed in FIG. 9) is enabled by virtue of the fact that the portion of the cylinder to the left of piston 144 is exhausted through line 140 to line 120 through the master pressure four-way control valve at the operators control panel. Cylinder 90 can be retracted by a reverse operation in an obvious manner.

Pilot check valve 130 is a conventional valve which is opened to permit exhaust flow through line 118 when the pressure in line 120 as communicated by line 132 to the pilot check valve is equal to or greater than a set value. In other words, pilot check valve 130 cannot exhaust unless the pressure in line 120 is equal to or greater than a desired minimum operating pressure. Similarly, pilot check valve 221 cannot exhaust through line 176 to valve 164 until the pressure in line 174 has reached or exceeded a set predetermined value.

It should be noted that the initial extension of cylinder 100 is easily enabled by virtue of the low-friction bearing members 70 etc. Moreover, the low friction slide bearing block 105 rides on the exterior of cylinder 100 which is smoothly polished for also aiding in achieving a minimum of frictional resistance to extension of cylinder 100.

FIG. 10 illustrates a second hydraulic control circuit which enables the simultaneous extension of cylinders and in a manner of operation that cannot be obtained with the control system illustrated in FIG. 9. The system of FIG. 10 differs from that of FIG. 9 solely in the inclusion of a pair of four-way three-position valves 300 and 302 each having a manually operable valve piston in a conventional manner. Valve 300 is connected to line and valve 302 is connected to line 1 18. Valve 300 has a valve piston 304 operable in a conventional manner and valve 302 has a valve piston 306 that is also operable in a conventional manner with both of the pistons being illustrated in FIG. 10. The positions of the valve pistons illustrated in FIG. 10 are neutral positions in which pressure from a pressure source 310 is returned to a schematically illustrated exhaust sump 312.

The system shown in FIG. 10 also requires that cylinder 90 must be fully retracted in order to operate cylinder 100 with positive control of extension movement. However, simultaneous extension of both cylinders can be accomplished by actuating valves 300 and 302 to simultaneously apply pressure to both sides of piston 144 in cylinder 90. The hydraulic pressure to the left of piston 144 acts upon an end cylinder wall 320 that is of smaller area than is the other end cylinder wall 322 by virtue of the fact that rod 106 extends through wall 320 and consequently decreases the wall area. Therefore, the differential in area between end cylinder walls 320 and 322 creates a force differential which would tend to extend cylinder 90. Valve 164 is simultaneously positioned so that pressure from lien 172 enters cylinder 100 through line 176 to extend cylinder 100 and line 174 is connected to line 166.

Therefore, it will be seen that the instant invention provides a uniquely compact space-saving arrangement in which a fully extendable boom formed of three boom sections is provided without the need for external cylinders, cables or the like. Many variations and modifications of the subject invention will occur to those skilled in the art; however, it should be understood that the spirit and scope of this invention should be interpreted solely in light of the appended claims.

We claim:

1. An extendable boom system comprising an inner boom section connected adjacent one end to a horizontal support pivot means for pivotal movement in a vertical plane, a hollow middle boom section telescopically received within and axially movable with respect to said inner boom section, a hollow outer boom section telescopically received within said middle boom section and axially movable with respect to said middle boom section, first and second hydraulic piston and cylinder assemblies mounted on the interior of said boom, said first piston and cylinder assembly comprising a first cylinder having a first axially reciprocal internal piston and a first piston rod extending from one end of said first cylinder, said second piston and cylinder assembly comprising a second cylinder having a second axially reciprocal internal piston and a second piston rod extending from one end of said second cylinder, means connecting said first piston rod to a point adjacent that innermost end of said inner boom section, means connecting said first cylinder to a point adjacent the innermost end of said middle boom section whereby actuation of said first hydraulic piston and cylinder assembly to extend said first hydraulic piston and cylinder assembly serves to move said middle boom section outwardly with respect to said inner boom section, means connecting said second piston to the inner end of said middle boom section, means connecting said second cylinder to the inner end of said outer boom section, whereby extension of said second hydraulic piston and cylinder assembly serves to move said outer boom section and hydraulic control means for selectively providing pressurized hydraulic fluid to said first and second hydraulic piston and cylinder assemblies for extending and retracting said boom.

2. The invention of claim 1 wherein said inner boom section, said middle section and said outer boom section are of rectangular cross-sectional configuration.

3. The invention of claim 2 additionally including slide block low-friction bearing means fixedly connected to the outer end of said first cylinder, and in sliding engagement with the exterior of said second cylinder for enabling relative movement of said cylinders with respect to each other with a minimum of friction and with maximum lateral positional stability.

4. The invention of claim 3 wherein said first piston rod includes a first internal rod passageway extending axially along the piston rod from the outer end of the piston rod externally of the first cylinder to a radial passageway communicating with the interior of the first cylinder on the rod side of the first cylinder interior and a second internal rod passageway extending the entire length of said first piston rod through said first piston to provide communication from the extreme outer end of said first piston rod externally of the first cylinder and that internal portion of the first cylinder on the side of said first piston opposite said first piston rod.

5. The invention of claim 4 additionally including low-friction cylindrical Teflon bearings between adjacent portions of said axially reciprocal boom sections for enabling low-friction movement of said boom sections between extended and retracted positions.

6. The invention of claim 4 wherein said second piston rod is provided with a first internal rod passageway extending axially along said second piston rod from the outer end of the second piston rod externally of the second cylinder to a radial passageway adjacent the second piston communicating with the interior of the second cylinder on the rod side of the cylinder interior and a second internal rod passageway extending the entire length of said second piston rod and through said second piston to provide communication between the extreme outer end of said second piston rod externally of said second cylinder and the portion of said second cylinder interior on the side of said second piston opposite said second piston rod.

7. The invention of claim 6 wherein said hydraulic control means includes a main cylinder connecting hydraulic line connected to the first cylinder on the rod end for communication with the portion of the interior of said first cylinder adjacent the piston rod end of said first cylinder and connected on its other end to a selectively operable directional control valve, a first second cylinder hydraulic power line extending between said directional control valve and the external rod end of said first internal rod passageway in said second piston rod, a second second cylinder hydraulic power line extending between said directional control valve and the external rod end of said second internal rod passageway in said second piston rod and an exhaust line extending between said directional control valve and the end of said first cylinder opposite the end of said first cylinder from which said first piston rod extends wherein said directional control valve is selectively operable to connect either of said second cylinder hydraulic power lines to said main cylinder connecting line while the other hydraulic power line is connected to said exhaust line and wherein said directional control valve is positionable in another position to simultaneously block said second cylinder hydraulic power lines to hold said second cylinder in a fixed position.

8. The invention of claim 7 wherein said hydraulic control means additionally includes first and second main hydraulic power inlet lines connected to said external ends of said first and second internal rod passageways of said first piston rod.

9. The invention of claim 8 wherein said directional control valve is a pilot signal operated spring centered piston valve and additionally including first and second pilot pressure lines extending from an operator controlled selectively operable valve for selectively connecting one of said pilot pressure lines to a source of pressure while the other is connected to an exhaust line or for connecting both of said pilot pressure lines simultaneously in fluid communication with each other so that said directional control valve assumes a spring biased blocking position in which said first second cylinder hydraulic power line and said second second cylinder hydraulic power line are blocked to hold said second piston and cylinder assembly in fixed position.

10. The invention of claim 8 wherein said first and second hydraulic power inlet lines are selectively connectable to a source of hydraulic pressure or to an exhaust sump through a four-way three-position master valve operable by the operator.

11. The invention of claim 8 wherein said first and second main hydraulic power inlet lines each include a selectively operable valve for connecting said line to a source of hydraulic pressure or to an exhaust with each of said valves selectively operable to connect both of said main hydraulic power inlet lines simultaneously to said source of hydraulic pressure.

12. The invention of claim 1 additionally including slide block low-friction bearing means fixedly connected to the outer end of said first cylinder and in sliding engagement with the exterior of said second cylinder for enabling relative movement of said cylinders with respect to each other with a minimum of friction and with maximum lateral positional stability.

13. The invention of claim 12 wherein said first piston rod includes a first internal rod passageway extending axially along the piston rod from the outer end of the piston rod externally of the first cylinder to a radial passageway communicating with the interior of the first cylinder on the rod side of the first cylinder interior and a second internal rod passageway extending the entire length of said first piston rod through said first piston to provide communication from the extreme outer end of said first piston rod externally of the first cylinder and that internal portion of the first cylinder on the side of said first piston opposite said first piston rod.

14. The invention of claim 13 wherein said second piston rod is provided with a first internal rod passageway extending axially along said second piston rod from the outer end of the second piston rod externally of the second cylinder to a radial passageway adjacent the second piston communicating with the interior of the second cylinder on the rod side of the cylinder interior and a second internal rod passageway extending the entire length of said second piston rod and through said second piston to provide communication between the extreme outer end of said second piston rod externally of said second cylinder and the portion of said second cylinder interior on the side of said second piston opposite said second piston rod.

15. The invention of claim 14 wherein said hydraulic control means includes a main cylinder connecting hydraulic line connected to the first cylinder on the rod end for communication with the portion of the interior of said first cylinder adjacent the piston rod end of said first cylinder and connected on its other end to a selectively operable directional control valve, a first second cylinder hydraulic power line extending between said directional control valve and the external rod end of said first internal rod passageway in said second piston rod, a second second cylinder hydraulic power line extending between said directional control valve and the external rod end of said second internal rod passageway in said second piston rod and an exhaust line extending between said directional control valve and the end of said first cylinder opposite the end of said first cylinder from which said first piston rod extends wherein said directional control valve is selectively operable to connect either of said second cylinder hydraulic power lines to said main cylinder connecting line while the other hydraulic power line is connected to said exhaust line and wherein said directional control valve is positionable in another position to simultaneously block said second cylinder hydraulic power lines to hold said second cylinder in a fixed position.

16. The invention of claim 15 wherein said hydraulic control means additionally includes first and second main hydraulic power inlet lines connected to said external ends of said first and second internal rod passageways of said first piston rod.

17. The invention of claim 16 wherein said directional control valve is a pilot signal operated spring centered piston valve and additionally including first and second pilot pressure lines extending from an operator controlled selectively operable valve for selectively connecting one of said pilot pressure lines to a source of pressure while the other is connected to an exhaust line or for connecting both of said pilot pressure lines simultaneously in fluid communication with each other so that said directional control valve assumes a spring biased blocking position in which said first second cylinder hydraulic power line and said second second cylinder hydraulic power line are blocked to hold said second piston and cylinder assembly in fixed position.

18. The invention of claim 17 wherein said first and second hydraulic power inlet lines are selectively connectable to a source of hydraulic pressure or to an exhaust sump through a four-way three-position master valve operable by the operator.

19. The invention of claim 18 wherein said first and second main hydraulic power inlet lines each include a selectively operable valve for connecting said line to a source of hydraulic pressure or to an exhaust with each of said valves in said first and second main hydraulic power inlet lines being selectively operable to connect both of said main hydraulic power inlet lines simultaneously to said source of hydraulic pressure.

20. The invention of claim 9 wherein said first and second pilot pressure lines are flexible hose members.

21. The invention of claim 20 wherein said directional control valve is fixedly connected to said middle boom on the innermost end thereof.

22. The invention of claim 21 wherein said first and second hydraulic power inlet lines are selectively connectable to a source of hydraulic pressure or to an exhaust sump through a four-way three-position master valve operable by the operatOl'. I

23. The invention of claim 22 wherein said first and second main hydraulic power inlet lines each include a selectively operable valve for connecting said lines to a source of hydraulic pressure or to an exhaust with each of said valves in said first and second main hydraulic power inlet lines being selectively operable to connect both of said main hydraulic power inlet lines simultaneously to said source of hydraulic pressure.

24. An extendable boom comprising an inner boom section, a hollow middle boom section and a hollow outer boom sec tion with the hollow outer boom section being telescopically received within said middle boom section which is telescopically received within said inner boom section when said boom is in a contracted condition, a first hydraulic cylinder containing a first piston and having a first piston rod extending from one end thereof, a second hydraulic cylinder containing a second piston and having a second piston rod extending from one end of said second cylinder, said first and second hydraulic cylinders being positioned adjacent each other in side-byside relationship when said boom is in its retracted position, means connecting said first piston rod to the innermost end of said inner boom, means connecting the innermost end of said first cylinder to the innermost end of said second boom, means connecting the end of said second piston rod to the innermost end of said second boom, means connecting the innermost end of said second cylinder to said outer boom section, hydraulic control means for providing pressurized hydraulic fluid for extending both of said cylinders to extend said boom to an extended position and slide bearing means connecting said first and second cylinders for permitting relative axial movement of said cylinders with respect to each other while retaining lateral stability between said cylinders during the extension of said boom.

25. The invention of claim 24 wherein said first piston rod includes a first internal rod passageway extending axially along the piston rod from the outer end of the piston rod externally of the first cylinder to a radial passageway communicating with the interior of the first cylinder on the rod side of the first cylinder interior and a second internal rod passageway extending the entire length of said first piston rod through said first piston to provide communication from the extreme outer end of said first piston rod externally of the first cylinder and that internal portion of the first cylinder on the side of said first piston opposite said first piston rod.

26. The invention of claim 25 wherein said second piston rod is provided with a first internal rod passageway extending axially along said second piston rod from the outer end of the second piston rod externally of the second cylinder to a radial passageway adjacent the second piston communicating with the interior of the second cylinder on the rod side of the cylinder interior and a second internal rod passageway extending the entire length of said second piston rod and through said second piston to provide communication between the extreme outer end of said second piston rod externally of said second cylinder and the portion of said second cylinder interior on the side of said second piston opposite said second piston rod.

27. The invention of claim 26 wherein said hydraulic control means includes a main cylinder connecting hydraulic line connected to the first cylinder on the rod end for communication with the portion of the interior of said first cylinder ad jacent the piston rod end of said first cylinder and connected on its other end to a selectively operable directional control valve, a first second cylinder hydraulic power line extending between said directional control valve and the external rod end of said first internal rod passageway in said second piston rod, a second second cylinder hydraulic power line extending between said directional control valve and the external rod end of said second internal rod passageway in said second piston rod and an exhaust line extending between said directional control valve and the end of said first cylinder opposite the end of said first cylinder from which said first piston rod extends wherein said directional control valve is selectively operable to connect either of said second cylinder hydraulic power lines to said main cylinder connecting line while the other hydraulic power line is connected to said exhaust line and wherein said directional control valve is positionable in another position to simultaneously block said second cylinder hydraulic power lines to hold said second cylinder in a fixed position.

28. The invention of claim 27 wherein said hydraulic control means additionally includes first and second main hydraulic power inlet lines connected to said external ends of said first and second internal rod passageways of said first piston rod.

29. The invention of claim 28 wherein said directional control valve is a pilot signal operated spring centered piston valve and additionally including first and second pilot pressure lines extending from an operator controlled selectively operable valve for selectively connecting one of said pilot pressure lines to a source of pressure while the other is connected to an exhaust line or for connecting both of said pilot pressure lines simultaneously in fluid communication with each other so that said directional control valve assumes a spring biased blocking position in which said first second cylinder hydraulic power line and said second second cylinder hydraulic power line are blocked to hold said second piston and cylinder assembly in fixed position.

30. The invention of claim 29 wherein said first and second hydraulic power inlet lines are selectively connectable to a source of hydraulic pressure or to an exhaust sump through a four-way three-position master valve operable by the operator.

31. The invention of claim 30 wherein said first and second main hydraulic power inlet lines each include a selectively operable valve for connecting said line to a source of hydraulic pressure or to an exhaust with each of said valves in said first and second main hydraulic power inlet lines being selectively operable to connect both of said main hydraulic power inlet lines simultaneously to said source of hydraulic pressure. 

1. An extendable boom system comprising an inner boom section connected adjacent one end to a horizontal support pivot means for pivotal movement in a vertical plane, a hollow middle boom section telescopically received within and axially movable with respect to said inner boom section, a hollow outer boom section telescopically received within said middle boom section and axially movable with respect to said middle boom section, first and second hydraulic piston and cylinder assemblies mounted on the interior of said boom, said first piston and cylinder assembly comprising a first cylinder having a first axially reciprocal internal piston and a first piston rod extending from one end of said first cylinder, said second piston and cylinder assembly comprising a second cylinder having a second axially reciprocal internal piston and a second piston rod extending from one end of said second cylinder, means connecting said first piston rod to a point adjacent that innermost end of said inner boom section, means connecting said first cylinder to a point adjacent the innermost end of said middle boom section whereby actuation of said first hydraulic piston and cylinder assembly to extend said first hydraulic piston and cylinder assembly serves to move said middle boom section outwardly with respect to said inner boom section, means connecting said second piston to the inner end of said middle boom section, means connecting said second cylinder to the inner end of said outer boom section, whereby extension of said second hydraulic piston and cylinder assembly serves to move said outer boom section and hydraulic control means for selectively providing pressurized hydraulic fluid to said first and second hydraulic piston and cylinder assemblies for extending and retracting said boom.
 2. The invention of claim 1 wherein said inner boom section, said middle section and said outer boom section are of rectangular cross-sectional configuration.
 3. The invention of claim 2 additionally including slide block low-friction bearing means fixedly connected to the outer end of said first cylinder, and in sliding engagement with the exterior of said second cylinder for enabling relative movement of said cylinders with respect to each other with a minimum of friction and with maximum lateral positional stability.
 4. The invention of claim 3 wherein said first piston rod includes a first internal rod passageway extending axially along the piston rod from the outer end of the piston rod externally of the first cylinder to a radial passageway communicating with the interior of the first cylinder on the rod side of the first cylinder interior and a second internal rod passageway extending the entire length of said first piston rod through said first piston to provide communication from the extreme outer end of said first piston rod externally of the first cylinder and that internal portion of the first cylinder on the side of said first piston opposite said first piston rod.
 5. The invention of claim 4 additionally including low-friction cylindrical Teflon bearings between adjacent portions of said axially reciprocal boom sections for enabling low-friction movement of said boom sections between extended and retracted poSitions.
 6. The invention of claim 4 wherein said second piston rod is provided with a first internal rod passageway extending axially along said second piston rod from the outer end of the second piston rod externally of the second cylinder to a radial passageway adjacent the second piston communicating with the interior of the second cylinder on the rod side of the cylinder interior and a second internal rod passageway extending the entire length of said second piston rod and through said second piston to provide communication between the extreme outer end of said second piston rod externally of said second cylinder and the portion of said second cylinder interior on the side of said second piston opposite said second piston rod.
 7. The invention of claim 6 wherein said hydraulic control means includes a main cylinder connecting hydraulic line connected to the first cylinder on the rod end for communication with the portion of the interior of said first cylinder adjacent the piston rod end of said first cylinder and connected on its other end to a selectively operable directional control valve, a first second cylinder hydraulic power line extending between said directional control valve and the external rod end of said first internal rod passageway in said second piston rod, a second second cylinder hydraulic power line extending between said directional control valve and the external rod end of said second internal rod passageway in said second piston rod and an exhaust line extending between said directional control valve and the end of said first cylinder opposite the end of said first cylinder from which said first piston rod extends wherein said directional control valve is selectively operable to connect either of said second cylinder hydraulic power lines to said main cylinder connecting line while the other hydraulic power line is connected to said exhaust line and wherein said directional control valve is positionable in another position to simultaneously block said second cylinder hydraulic power lines to hold said second cylinder in a fixed position.
 8. The invention of claim 7 wherein said hydraulic control means additionally includes first and second main hydraulic power inlet lines connected to said external ends of said first and second internal rod passageways of said first piston rod.
 9. The invention of claim 8 wherein said directional control valve is a pilot signal operated spring centered piston valve and additionally including first and second pilot pressure lines extending from an operator controlled selectively operable valve for selectively connecting one of said pilot pressure lines to a source of pressure while the other is connected to an exhaust line or for connecting both of said pilot pressure lines simultaneously in fluid communication with each other so that said directional control valve assumes a spring biased blocking position in which said first second cylinder hydraulic power line and said second second cylinder hydraulic power line are blocked to hold said second piston and cylinder assembly in fixed position.
 10. The invention of claim 8 wherein said first and second hydraulic power inlet lines are selectively connectable to a source of hydraulic pressure or to an exhaust sump through a four-way three-position master valve operable by the operator.
 11. The invention of claim 8 wherein said first and second main hydraulic power inlet lines each include a selectively operable valve for connecting said line to a source of hydraulic pressure or to an exhaust with each of said valves selectively operable to connect both of said main hydraulic power inlet lines simultaneously to said source of hydraulic pressure.
 12. The invention of claim 1 additionally including slide block low-friction bearing means fixedly connected to the outer end of said first cylinder and in sliding engagement with the exterior of said second cylinder for enabling relative movement of said cylinders with respect to each other with a minimum of Friction and with maximum lateral positional stability.
 13. The invention of claim 12 wherein said first piston rod includes a first internal rod passageway extending axially along the piston rod from the outer end of the piston rod externally of the first cylinder to a radial passageway communicating with the interior of the first cylinder on the rod side of the first cylinder interior and a second internal rod passageway extending the entire length of said first piston rod through said first piston to provide communication from the extreme outer end of said first piston rod externally of the first cylinder and that internal portion of the first cylinder on the side of said first piston opposite said first piston rod.
 14. The invention of claim 13 wherein said second piston rod is provided with a first internal rod passageway extending axially along said second piston rod from the outer end of the second piston rod externally of the second cylinder to a radial passageway adjacent the second piston communicating with the interior of the second cylinder on the rod side of the cylinder interior and a second internal rod passageway extending the entire length of said second piston rod and through said second piston to provide communication between the extreme outer end of said second piston rod externally of said second cylinder and the portion of said second cylinder interior on the side of said second piston opposite said second piston rod.
 15. The invention of claim 14 wherein said hydraulic control means includes a main cylinder connecting hydraulic line connected to the first cylinder on the rod end for communication with the portion of the interior of said first cylinder adjacent the piston rod end of said first cylinder and connected on its other end to a selectively operable directional control valve, a first second cylinder hydraulic power line extending between said directional control valve and the external rod end of said first internal rod passageway in said second piston rod, a second second cylinder hydraulic power line extending between said directional control valve and the external rod end of said second internal rod passageway in said second piston rod and an exhaust line extending between said directional control valve and the end of said first cylinder opposite the end of said first cylinder from which said first piston rod extends wherein said directional control valve is selectively operable to connect either of said second cylinder hydraulic power lines to said main cylinder connecting line while the other hydraulic power line is connected to said exhaust line and wherein said directional control valve is positionable in another position to simultaneously block said second cylinder hydraulic power lines to hold said second cylinder in a fixed position.
 16. The invention of claim 15 wherein said hydraulic control means additionally includes first and second main hydraulic power inlet lines connected to said external ends of said first and second internal rod passageways of said first piston rod.
 17. The invention of claim 16 wherein said directional control valve is a pilot signal operated spring centered piston valve and additionally including first and second pilot pressure lines extending from an operator controlled selectively operable valve for selectively connecting one of said pilot pressure lines to a source of pressure while the other is connected to an exhaust line or for connecting both of said pilot pressure lines simultaneously in fluid communication with each other so that said directional control valve assumes a spring biased blocking position in which said first second cylinder hydraulic power line and said second second cylinder hydraulic power line are blocked to hold said second piston and cylinder assembly in fixed position.
 18. The invention of claim 17 wherein said first and second hydraulic power inlet lines are selectively connectable to a source of hydraulic pressure or to an exhaust sump through a four-way three-positiOn master valve operable by the operator.
 19. The invention of claim 18 wherein said first and second main hydraulic power inlet lines each include a selectively operable valve for connecting said line to a source of hydraulic pressure or to an exhaust with each of said valves in said first and second main hydraulic power inlet lines being selectively operable to connect both of said main hydraulic power inlet lines simultaneously to said source of hydraulic pressure.
 20. The invention of claim 9 wherein said first and second pilot pressure lines are flexible hose members.
 21. The invention of claim 20 wherein said directional control valve is fixedly connected to said middle boom on the innermost end thereof.
 22. The invention of claim 21 wherein said first and second hydraulic power inlet lines are selectively connectable to a source of hydraulic pressure or to an exhaust sump through a four-way three-position master valve operable by the operator.
 23. The invention of claim 22 wherein said first and second main hydraulic power inlet lines each include a selectively operable valve for connecting said lines to a source of hydraulic pressure or to an exhaust with each of said valves in said first and second main hydraulic power inlet lines being selectively operable to connect both of said main hydraulic power inlet lines simultaneously to said source of hydraulic pressure.
 24. An extendable boom comprising an inner boom section, a hollow middle boom section and a hollow outer boom section with the hollow outer boom section being telescopically received within said middle boom section which is telescopically received within said inner boom section when said boom is in a contracted condition, a first hydraulic cylinder containing a first piston and having a first piston rod extending from one end thereof, a second hydraulic cylinder containing a second piston and having a second piston rod extending from one end of said second cylinder, said first and second hydraulic cylinders being positioned adjacent each other in side-by-side relationship when said boom is in its retracted position, means connecting said first piston rod to the innermost end of said inner boom, means connecting the innermost end of said first cylinder to the innermost end of said second boom, means connecting the end of said second piston rod to the innermost end of said second boom, means connecting the innermost end of said second cylinder to said outer boom section, hydraulic control means for providing pressurized hydraulic fluid for extending both of said cylinders to extend said boom to an extended position and slide bearing means connecting said first and second cylinders for permitting relative axial movement of said cylinders with respect to each other while retaining lateral stability between said cylinders during the extension of said boom.
 25. The invention of claim 24 wherein said first piston rod includes a first internal rod passageway extending axially along the piston rod from the outer end of the piston rod externally of the first cylinder to a radial passageway communicating with the interior of the first cylinder on the rod side of the first cylinder interior and a second internal rod passageway extending the entire length of said first piston rod through said first piston to provide communication from the extreme outer end of said first piston rod externally of the first cylinder and that internal portion of the first cylinder on the side of said first piston opposite said first piston rod.
 26. The invention of claim 25 wherein said second piston rod is provided with a first internal rod passageway extending axially along said second piston rod from the outer end of the second piston rod externally of the second cylinder to a radial passageway adjacent the second piston communicating with the interior of the second cylinder on the rod side of the cylinder interior and a second internal rod passageway extending the entire length of said second piston rod and thrOugh said second piston to provide communication between the extreme outer end of said second piston rod externally of said second cylinder and the portion of said second cylinder interior on the side of said second piston opposite said second piston rod.
 27. The invention of claim 26 wherein said hydraulic control means includes a main cylinder connecting hydraulic line connected to the first cylinder on the rod end for communication with the portion of the interior of said first cylinder adjacent the piston rod end of said first cylinder and connected on its other end to a selectively operable directional control valve, a first second cylinder hydraulic power line extending between said directional control valve and the external rod end of said first internal rod passageway in said second piston rod, a second second cylinder hydraulic power line extending between said directional control valve and the external rod end of said second internal rod passageway in said second piston rod and an exhaust line extending between said directional control valve and the end of said first cylinder opposite the end of said first cylinder from which said first piston rod extends wherein said directional control valve is selectively operable to connect either of said second cylinder hydraulic power lines to said main cylinder connecting line while the other hydraulic power line is connected to said exhaust line and wherein said directional control valve is positionable in another position to simultaneously block said second cylinder hydraulic power lines to hold said second cylinder in a fixed position.
 28. The invention of claim 27 wherein said hydraulic control means additionally includes first and second main hydraulic power inlet lines connected to said external ends of said first and second internal rod passageways of said first piston rod.
 29. The invention of claim 28 wherein said directional control valve is a pilot signal operated spring centered piston valve and additionally including first and second pilot pressure lines extending from an operator controlled selectively operable valve for selectively connecting one of said pilot pressure lines to a source of pressure while the other is connected to an exhaust line or for connecting both of said pilot pressure lines simultaneously in fluid communication with each other so that said directional control valve assumes a spring biased blocking position in which said first second cylinder hydraulic power line and said second second cylinder hydraulic power line are blocked to hold said second piston and cylinder assembly in fixed position.
 30. The invention of claim 29 wherein said first and second hydraulic power inlet lines are selectively connectable to a source of hydraulic pressure or to an exhaust sump through a four-way three-position master valve operable by the operator.
 31. The invention of claim 30 wherein said first and second main hydraulic power inlet lines each include a selectively operable valve for connecting said line to a source of hydraulic pressure or to an exhaust with each of said valves in said first and second main hydraulic power inlet lines being selectively operable to connect both of said main hydraulic power inlet lines simultaneously to said source of hydraulic pressure. 